It is well known to provide electronic article surveillance (EAS) systems to prevent or deter theft of merchandise from retail establishments. In a typical system, markers designed to interact with a magnetic field placed at the store exit are secured to articles of merchandise. If a marker is brought into the field or "surveillance zone," the presence of the marker is detected and an alarm is generated.
One type of magnetic EAS system is referred to as a harmonic system because it is based on the principle that a magnetic material passing through an electromagnetic field having a selected frequency disturbs the field and produces harmonic perturbations of the selected frequency. The detection system is tuned to recognize certain harmonic frequencies and, if present, causes an alarm.
A basic problem in the design of markers for harmonic EAS systems is the need to have the marker generate a harmonic signal that is both of sufficient amplitude to be readily detectable and also is sufficiently unique so that the detection equipment can be tuned to detect only the signal generated by the marker, while disregarding harmonic disturbances caused by the presence of items such as coins, keys, and so forth. A known approach to this problem is to develop markers that produce high order harmonics with sufficient amplitude to be readily detectable. A particularly useful technique along these lines is disclosed in U.S. Pat. No. 4,660,025, issued to Humphrey, the disclosure of which is incorporated herein by reference. The Humphrey patent discloses a harmonic EAS marker employing as its active element a wire of magnetic material which has a magnetic hysteresis loop with a large discontinuity, known as a "Barkhausen discontinuity."
A marker of the type disclosed in the Humphrey patent is shown in FIG. 1. The marker, designated generally by the reference number 10, consists of an active element 11, in the form of a wire of magnetic material, sandwiched between a substrate 12 and a overlayer 13. Typically, an adhesive is provided on the lower surface of the substrate 12 for use in affixing the marker 10 to an article of merchandise (not shown).
The wire 11 is of the type referred to as "re-entrant;"that is, it exhibits a magnetic hysteresis loop, as shown in FIG. 2, characterized by Barkhausen discontinuities, represented by broken lines 14. As a result, upon exposure to an alternating magnetic field of sufficient amplitude, the wire 11 undergoes substantially instantaneous regenerative reversals in magnetic polarity, producing very sharp signal spikes that are rich in detectable high harmonics of the frequency of the alternating field.
Markers employing the type of active element just described have been very successfully placed in practice, and are in widespread use with harmonic EAS systems distributed by the assignee of the present application under the trademark "AISLEKEEPER".
One design objective that has so far only been partially realized is reduction in length of markers employing re-entrant wires. Re-entrant wire markers currently in use have lengths of about 65 or 90 mm. It would be desirable to provide a harmonic EAS marker substantially shorter than 65 mm for use with relatively small articles of merchandise and/or for incorporation in price marking labels. One constraint upon reducing the length of the re-entrant wires is that large Barkhausen discontinuities can only be produced in active elements having a high ratio of length to cross-sectional area to provide a very low demagnetizing factor. Die-drawn re-entrant wires having a length of 65 mm have been used successfully, but shorter, thinner wires, and re-entrant materials formed as thin films, are very low in mass, and, therefore, generate signals that are too low in amplitude for reliable detection.
It can be contemplated to form a marker using two or more short, thin wires arranged in parallel in order to obtain a higher output amplitude. However, it has been found that the wires do not simultaneously switch polarities in response to the alternating field, and thus fail to provide a signal of the desired amplitude.